from __future__ import annotations
import sys
import traceback
from typing import Any, Callable, Generic, Optional, Set, TypeVar, cast
from weakref import WeakKeyDictionary, ref
P = TypeVar("P")
class _Obs(Generic[P]):
"""
Internal holder for Property value and change listeners
"""
__slots__ = ("value", "listeners")
def __init__(self, value: P):
self.value = value
# This will keep any added listener even if it is not referenced anymore and would be garbage collected
self.listeners: Set[Callable[[Any, P], Any]] = set()
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class Property(Generic[P]):
"""
An observable property which triggers observers when changed.
.. code-block:: python
def log_change(instance, value):
print("Something changed")
class MyObject:
name = Property()
my_obj = MyObject()
bind(my_obj, "name", log_change)
unbind(my_obj, "name", log_change)
my_obj.name = "Hans"
# > Something changed
:param default: Default value which is returned, if no value set before
:param default_factory: A callable which returns the default value.
Will be called with the property and the instance
"""
__slots__ = ("name", "default_factory", "obs")
name: str
def __init__(
self,
default: Optional[P] = None,
default_factory: Optional[Callable[[Any, Any], P]] = None,
):
if default_factory is None:
default_factory = lambda prop, instance: cast(P, default)
self.default_factory = default_factory
self.obs: WeakKeyDictionary[Any, _Obs] = WeakKeyDictionary()
def _get_obs(self, instance) -> _Obs:
obs = self.obs.get(instance)
if obs is None:
obs = _Obs(self.default_factory(self, instance))
self.obs[instance] = obs
return obs
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def get(self, instance) -> P:
obs = self._get_obs(instance)
return obs.value
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def set(self, instance, value):
obs = self._get_obs(instance)
if obs.value != value:
obs.value = value
self.dispatch(instance, value)
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def dispatch(self, instance, value):
obs = self._get_obs(instance)
for listener in obs.listeners:
try:
try:
listener(instance, value)
except TypeError:
# If the listener does not accept arguments, we call it without it
listener() # type: ignore
except Exception:
print(
f"Change listener for {instance}.{self.name} = {value} raised an exception!",
file=sys.stderr,
)
traceback.print_exc()
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def bind(self, instance, callback):
obs = self._get_obs(instance)
# Instance methods are bound methods, which can not be referenced by normal `ref()`
# if listeners would be a WeakSet, we would have to add listeners as WeakMethod ourselves into `WeakSet.data`.
obs.listeners.add(callback)
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def unbind(self, instance, callback):
obs = self._get_obs(instance)
obs.listeners.remove(callback)
def __set_name__(self, owner, name):
self.name = name
def __get__(self, instance, owner) -> P:
if instance is None:
return self # type: ignore
return self.get(instance)
def __set__(self, instance, value):
self.set(instance, value)
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def bind(instance, property: str, callback):
"""
Binds a function to the change event of the property. A reference to the function will be kept,
so that it will be still invoked, even if it would normally have been garbage collected.
def log_change(instance, value):
print(f"Value of {instance} changed to {value}")
class MyObject:
name = Property()
my_obj = MyObject()
bind(my_obj, "name", log_change)
my_obj.name = "Hans"
# > Value of <__main__.MyObject ...> changed to Hans
:param instance: Instance owning the property
:param property: Name of the property
:param callback: Function to call
:return: None
"""
t = type(instance)
prop = getattr(t, property)
if isinstance(prop, Property):
prop.bind(instance, callback)
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def unbind(instance, property: str, callback):
"""
Unbinds a function from the change event of the property.
def log_change(instance, value):
print("Something changed")
class MyObject:
name = Property()
my_obj = MyObject()
bind(my_obj, "name", log_change)
unbind(my_obj, "name", log_change)
my_obj.name = "Hans"
# > Something changed
:param instance: Instance owning the property
:param property: Name of the property
:param callback: Function to unbind
:return: None
"""
t = type(instance)
prop = getattr(t, property)
if isinstance(prop, Property):
prop.unbind(instance, callback)
class _ObservableDict(dict):
"""Internal class to observe changes inside a native python dict."""
__slots__ = ("prop", "obj")
def __init__(self, prop: Property, instance, *largs):
self.prop: Property = prop
self.obj = ref(instance)
super().__init__(*largs)
def dispatch(self):
self.prop.dispatch(self.obj(), self)
def __setitem__(self, key, value):
dict.__setitem__(self, key, value)
self.dispatch()
def __delitem__(self, key):
dict.__delitem__(self, key)
self.dispatch()
def clear(self):
dict.clear(self)
self.dispatch()
def pop(self, *largs):
result = dict.pop(self, *largs)
self.dispatch()
return result
def popitem(self):
result = dict.popitem(self)
self.dispatch()
return result
def setdefault(self, *largs):
dict.setdefault(self, *largs)
self.dispatch()
def update(self, *largs):
dict.update(self, *largs)
self.dispatch()
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class DictProperty(Property):
"""
Property that represents a dict.
Only dict are allowed. Any other classes are forbidden.
"""
def __init__(self):
super().__init__(default_factory=_ObservableDict)
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def set(self, instance, value: dict):
value = _ObservableDict(self, instance, value)
super().set(instance, value)
class _ObservableList(list):
"""Internal class to observe changes inside a native python list."""
__slots__ = ("prop", "obj")
def __init__(self, prop: Property, instance, *largs):
self.prop: Property = prop
self.obj = ref(instance)
super().__init__(*largs)
def dispatch(self):
self.prop.dispatch(self.obj(), self)
def __setitem__(self, key, value):
list.__setitem__(self, key, value)
self.dispatch()
def __delitem__(self, key):
list.__delitem__(self, key)
self.dispatch()
def __iadd__(self, *largs): # type: ignore
list.__iadd__(self, *largs)
self.dispatch()
return self
def __imul__(self, *largs): # type: ignore
list.__imul__(self, *largs)
self.dispatch()
return self
def append(self, *largs):
list.append(self, *largs)
self.dispatch()
def clear(self):
list.clear(self)
self.dispatch()
def remove(self, *largs):
list.remove(self, *largs)
self.dispatch()
def insert(self, *largs):
list.insert(self, *largs)
self.dispatch()
def pop(self, *largs):
result = list.pop(self, *largs)
self.dispatch()
return result
def extend(self, *largs):
list.extend(self, *largs)
self.dispatch()
def sort(self, **kwargs):
list.sort(self, **kwargs)
self.dispatch()
def reverse(self):
list.reverse(self)
self.dispatch()
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class ListProperty(Property):
"""
Property that represents a list.
Only list are allowed. Any other classes are forbidden.
"""
def __init__(self):
super().__init__(default_factory=_ObservableList)
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def set(self, instance, value: dict):
value = _ObservableList(self, instance, value) # type: ignore
super().set(instance, value)